1. Field of the Invention
The present invention relates to an optical writing unit in which a light-emitting-element array such as a light-emitting-diode (LED) array is used, and an image writing apparatus that includes the optical writing unit, and more particularly, to an image writing apparatus that enables to print a line image of inclined lines with fidelity, and to achieve further improvement in an aspect ratio of an image.
2. Description of the Related Art
Generally, a laser scanning method by laser beam is used in an image writing apparatus that writes a latent image by irradiating light on a photosensitive drum. A light-emitting-element array method in which LEDs are used in an array form, is one of the LD scanning methods.
In the light-emitting-element array method, when a binary image is output from an image forming apparatus, according to process conditions, a one-dot print is printed in an elliptical form that is wide horizontally. If it is a lattice image of one dot (an image at 5 mm interval), it appears more clearly, and a vertical line is printed thicker than a horizontal line and an aspect ratio becomes a problem.
To solve this problem, there is a method of controlling by using balance-correction data of an LED. In other words, gradation data for each LED, data that corrects output unevenness in units of a blocks that include a plurality of LEDs, and data that corrects output unevenness with respect to output unevenness for an average value of blocks are added together, and an attempt is made to suppress the unevenness of LED by data that is added. However, this method is for multi-valued data and a control method for reproducing with fidelity the gradation by adding data of a binary image and correction data of each LED, is available as a method in binary. However, since printing power (print drive current control) is adjusted, there is an improvement in a line image, but there is no improvement in vertical and horizontal line width. Moreover, according to the LED light-emitting-element array, there are some in which a method of adding the correction data and print image data are added, is not used.
In view of this, in a conventional LED light-emitting-element array, the control is performed by allowing lighting up for a number of times. However, in a light-emitting-element array unit that transfers odd pixel data upon transferring even pixel data from image data, it lights up twice for one image transfer.
As a conventional technology, according to Japanese Patent. Application Laid-open Publication No. H9-11540, gradation print data upon the unevenness correction is stored in a shift register 12, and bit sliced data is fetched by using a shift register 22 and AND gates A1 to A5. In synchronization with this, a reference current is changed by changing reference resistances R1 to R5 by a shift register 24 and switches S1 to S5.
According to the conventional method, it is possible to thicken width of the horizontal line and to improve the aspect ratio, but since there is a difference in even pixels and odd pixels in a line, a line image of an inclined line is serrated.